Biology Department, University of Vermont, Burlington, Vermont 05465, USA.
Ecology. 2010 Feb;91(2):399-407. doi: 10.1890/09-0004.1.
In African savannas, vertebrate herbivores are often identified as key determinants of plant growth, survivorship, and reproduction. However, plant reproduction is likely to be the product of responses to a suite of abiotic and biotic factors, including nutrient availability and interactions with antagonists and mutualists. In a relatively simple system, we examined the role of termites (which act as ecosystem engineers--modifying physical habitat and creating islands of high soil fertility), vertebrate herbivores, and symbiotic ants, on the fruiting success of a dominant plant, Acacia drepanolobium, in East African savannas. Using observational data, large-scale experimental manipulations, and analysis of foliar N, we found that Acacia drepanolobium trees growing at the edge of termite mounds were more likely to reproduce than those growing farther away, in off-mound soils. Although vertebrate herbivores preferentially used termite mounds as demonstrated by dung deposits, long-term exclusion of mammalian grazers did not significantly reduce A. drepanolobium fruit production. Leaf N was significantly greater in trees growing next to mounds than in those growing farther away, and this pattern was unaffected by exclusion of vertebrates. Thus, soil enrichment by termites, rather than through dung and urine deposition by large herbivores, is of primary importance to fruit production near mounds. Across all mound-herbivore treatment combinations, trees that harbored Crematogaster sjostedti were more likely to fruit than those that harbored one of the other three ant species. Although C. sjostedti is less aggressive than the other ants, it tends to inhabit large, old trees near termite mounds which are more likely to fruit than smaller ones. Termites play a key role in generating patches of nutrient-rich habitat important to the reproductive success of A. drepanolobium in East African savannas. Enhanced nutrient acquisition from termite mounds appears to allow plants to tolerate herbivory and the reduced defense by a relatively ineffective ant partner. Our results underscore the importance of simultaneously examining top-down and bottom-up effects to understand those factors most important to plant reproductive success.
在非洲热带稀树草原中，脊椎动物草食动物通常被认为是植物生长、存活和繁殖的关键决定因素。然而，植物繁殖可能是对一系列非生物和生物因素（包括养分供应和与拮抗剂和共生体的相互作用）的反应的产物。在一个相对简单的系统中，我们研究了白蚁（作为生态系统工程师——改变物理栖息地并创造高土壤肥力的岛屿）、脊椎动物草食动物和共生蚂蚁对东非热带稀树草原中优势植物金合欢生长的果实成功的作用。利用观测数据、大规模实验处理和叶片氮分析，我们发现，与远离白蚁丘的土壤相比，生长在白蚁丘边缘的金合欢树更有可能繁殖。尽管哺乳动物食草动物更喜欢在白蚁丘上活动，这一点可以从粪便沉积中得到证明，但长期排除食草动物并没有显著减少金合欢果实的产量。与远离白蚁丘的树木相比，生长在白蚁丘附近的树木叶片氮含量显著更高，而这种模式不受脊椎动物的影响。因此，白蚁对土壤的富化作用，而不是通过大型食草动物的粪便和尿液沉积，对靠近白蚁丘的果实生产至关重要。在所有的白蚁-食草动物处理组合中，栖息着 Crematogaster sjostedti 的树木比栖息着其他三种蚂蚁之一的树木更有可能结果。虽然 C. sjostedti 不如其他蚂蚁具有攻击性，但它往往栖息在靠近白蚁丘的大型、古老的树木中，这些树木比小型树木更容易结果。白蚁在东非热带稀树草原中产生富含营养的栖息地斑块方面发挥着关键作用，这些斑块对金合欢的繁殖成功至关重要。从白蚁丘中获得增强的养分获取能力似乎使植物能够耐受草食动物的侵害，并减轻相对无效的蚂蚁伙伴的防御。我们的研究结果强调了同时考察自上而下和自下而上的影响以了解对植物繁殖成功最重要的因素的重要性。
